Oil and gas wells have in general three different purposes, as producers of hydrocarbons, injectors of water or gas for reservoir pressure support or for depositing purposes, or as exploration wells. At some point it is likely to be necessary to satisfactorily plug and seal these wells, e.g. after the wells have reached their end-of life and it is not economically feasible to keep the wells in service (so-called “plug and abandon”), or for some temporary purpose (e.g. “slot recovery”). Plugging of wells is performed in connection with permanent abandonment of wells due to decommissioning of fields or in connection with permanent abandonment of a section of well to construct a new wellbore (known as side tracking or slot recovery) with a new geological well target.
A well is constructed by a hole being drilled down into the reservoir using a drilling rig and then sections of steel pipe, casing or liner are placed in the hole to impart structural integrity to the wellbore. Cement is placed between the outside of the casing or liner and the bore hole and then tubing is inserted into the casing to connect the wellbore to the surface. For ease of reference, all of these entities inserted into the well are referred to here as “tubulars”. When the reservoir is to be abandoned, either temporarily or permanently, a well barrier must be established across the full cross-section of the well. This is generally achieved by removal of the tubulars from the well bore by pulling the tubulars to the surface or by section milling. Well barriers are then established across the full cross-section of the well, in order to isolate the reservoir(s) and prevent flow of formation fluids between reservoirs or to the surface. It is necessary to remove the tubulars from the wellbore in the case that proper quality of the sealant (e.g. cement) behind the tubular(s) cannot be determined.
To save having to remove an entire length of tubular from a well, a tool may be inserted into the well to cut the tubulars at a point beneath that at which the plug is to be formed, and only the upper detached part of the tubulars removed from the well. It is also possible to use a milling tool to mill away a part of the tubulars at the location where the plug is to be formed.
Attempts have been made to increase the efficiency of the method of abandonment. For example, GB2407835 describes wellbore sealing wherein explosive charges are used to perforate a lower end of the tubing and then sealing fluid is pumped through the perforations so as to plug the well around the bottom end of the tubing. A similar approach is described in WO2012096580.
U.S. Pat. No. 2,591,807 relates to an apparatus that uses relatively low and high velocity explosive charges spaced at opposing ends of a container full of cement for placing in a zone of a wellbore whereby, upon ignition, cement is forced downwardly and outwardly to release cement into the cavity between the tubing and formation. U.S. Pat. No. 2,696,258 and U.S. Pat. No. 2,696,259 relate to an apparatus for depositing cement in a zone wherein the cement is contained within an elongated container and a gas generating charge is ignited to displace the cement through a lower outlet of the container into the zone. The charge expands the container into sealing contact with the casing, while at the same time rupturing the end of a tubular body to release cement into the wellbore.
Regulations may require that an abandoned well be plugged so as to seal the well over at least some specified longitudinal extent, e.g. greater than 50 meters. An improperly abandoned well is a serious liability so it is important to ensure that the well is adequately plugged and sealed. However, as it can be difficult to accurately determine the quality of a well plug, regulations will typically over specify plug requirements by some significant margin. In any case, even when a plug meets the specified requirements there may be a risk of failure for any number of reasons.
In order to determine the integrity of a well plug it is desirable to perform pressure testing. This is relatively easy to achieve from above the plug. However, it is currently not possible to perform pressure testing of a plug from below.